Optical evaluation of lenses and lens molds

ABSTRACT

A method for determining information about a transparent optical element including a lens portion and a plane parallel portion, the lens portion having at least one curved surface and the plane parallel portion having opposing first and second surfaces, includes: directing measurement light to the transparent optical element; detecting measurement light reflected from at least one location on the first surface of the plane parallel portion; detecting measurement light reflected from the second surface of the plane parallel portion at a location corresponding to the at least one location on the first surface; determining, based on the detected light, information about the plane parallel portion; and evaluating the transparent optical element based on the information about the plane parallel portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Application No.62/037,966, entitled “OPTICAL EVALUATION OF THE DIMENSIONAL AND OPTICALPROPERTIES OF LENSES HAVING MECHANICAL LOCATING FEATURES,” filed on Aug.15, 2014 and to Provisional Application No. 62/039,398, entitled“OPTICAL EVALUATION OF FEATURE LOCATIONS ON LENSES,” filed on Aug. 19,2014. The contents of both provisional applications is herebyincorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

In certain aspects, this disclosure relates to methods and tools forcharacterizing the geometry and optical properties of molded lenses usedin consumer products, as well as the diamond turned molds used for theirfabrication. This disclosure further relates to fabrication of opticalassemblies containing such molded lenses, and fabrication of consumerproducts containing the optical assemblies.

BACKGROUND

The market for miniaturized cameras used in smartphones and cellphones,tablets, portable computers, cars and trucks is rapidly expanding. Imagequality requirements for state-of-the-art cameras force manufacturers todevelop complex optical assemblies composed of many aspherical moldedlenses.

FIG. 1 shows an exemplary optical assembly composed of four moldedplastic lenses. In particular, this assembly, which is described in U.S.Pat. No. 7,777,972, includes four lenses arranged to form an image on asensor located at an image plane 170 of the assembly. The lens elementsare arranged in a first lens group composed of a first lens element 100with positive refractive power having an aspheric convex object-sidesurface 101 and an aspheric convex image-side surface 102. The assemblyalso includes a second lens group composed of a second lens element 110,a third lens element 120, and a fourth lens element 130.

The second lens element 110 has negative refractive power having anaspheric convex object-side surface 111 and an aspheric concaveimage-side surface 112. The third lens element 120 has an asphericconcave object-side surface 121 and an aspheric convex image-sidesurface 122. The fourth lens element 130 has an aspheric convexobject-side surface 131 and an aspheric concave image-side surface 132.An aperture stop 140 is positioned between the first lens element 100and the imaged object. An IR filter 150 is disposed between theimage-side surface 132 of the fourth lens element 130 and the imageplane 170, the IR filter 150 having no influence on the focal length ofthe imaging optical lens assembly. A sensor cover glass 160 is arrangedbetween the IR filter 150 and the image plane 170, the sensor coverglass 160 also having no influence on the focal length of the imagingoptical lens assembly.

In general, the curved lens surfaces are rotationally symmetrical aboutan axis, and each surface's axis nominally lies on a common axis—theoptical axis—of the assembly. Common centration of the lens surface axesis important for the optical performance of the overall assembly. Alsoimportant is the curvature of each lens surface and the spacing betweeneach lens surface—i.e., both the lens thickness and the spacing betweenadjacent lenses.

Accordingly, each individual lens typically includes multiple centrationand spacing datums, manufactured with tight enough tolerances to provideproper alignment of the final lens assembly when stacked together inaddition to the curved functional optical surfaces depicted in FIG. 1.These datums are commonly provided by additional non-optically activeportions of each lens, which form a ring around the edge of the activelens portions. When assembled, the non-optical portions of the lensesstack together, aligning and spacing the lens portions relative to eachother as required by the overall lens assembly design.

Because of tightening manufacturing tolerance budgets, conventionalmetrology equipment (e.g., contact probes and gages, tactile profilers,inspection microscopes) is in many cases no longer capable of achievingrequired measurement reproducibility or accuracy. Additionally,metrology equipment for measuring certain properties of the lenses, suchas refractive index or birefringence, on the production floor, is notcommercially available. Accordingly, a metrology gap exists today.

SUMMARY

The disclosure features methods and apparatus for evaluating thedimensional and optical properties of transparent samples, including inparticular, lenses including active surface areas that are curved, andalso including upper and lower surface areas that are nominally flat andparallel, for example for mechanically locating these lenses in anassembly.

In embodiments, the apparatus includes an optical metrology system formeasuring the relative location of the nominally parallel upper andlower surface areas of the sample, and a data processing system forevaluating the optical and dimensional properties of the sample usingthe information derived from the calculated optical and physicalthickness at two or more positions of the nominally parallel upper andlower surface areas of the sample.

Alternatively, or in addition, the disclosure features methods andapparatus for evaluating the dimensional properties of a transparentsample (e.g., a lens) by combining 3D surface topography information(e.g., a height profile) with 2D images (e.g., an intensity profile) forboth the upper and lower surfaces of the sample. The topographyinformation and the two images may be acquired from the same side of thesample.

The methods and apparatus may be used in a production environment.

In general, the sample may be a lens that includes upper and loweractive surface areas that are curved (e.g., having optical power), aswell as additional upper and lower surface areas that are not used fordirecting light, but rather for mechanically locating these lenses in anassembly. These portions may have plane parallel surfaces. The curvedportions may be referred to as active portions and the other (e.g.,plane parallel) portions as inactive portions. The sample may havesurface areas of the active portions that are convex or concave,spherical or aspheric.

The upper and lower surfaces of the inactive portions may be nominallyflat or conical, and may include nominally circular features orboundaries which may be nominally concentric with the apex of the activesurface area.

The sample may be a lens for use in a portable electronic device cameraassembly.

In general, the dimensional properties may include the location of theapex of an active surface area with respect to a datum feature (e.g., anominally circular datum feature) of an additional surface area. In someembodiments, the lens apex is found by evaluation of a 3D areal surfacetopography map, whereas the datum feature is located in a 2D image ofthe part. The 2D image may be extracted from the same data acquisitionas the 3D map, or may be part of a separate step.

The dimensional properties may include the relative height of upper andlower apex features of the sample. This measurement may rely onadditional information derived from the calculated optical and physicalthickness at two or more positions of the nominally parallel upper andlower surface areas of the sample.

In certain embodiments, the methodology includes compensation for therefractive properties of the lens when viewing features through thesample to determine their lateral position.

In some embodiments, the apparatus includes a part fixture that includesan auxiliary reference surface. This auxiliary reference surface may benominally flat, and may be located at a distance under the opticalcomponent under test in such a way that it reflects light thatpropagated through the optical component back through the component andtoward the metrology-device detector channel.

In some embodiments, the apparatus includes a part fixture that has boththe auxiliary reference surface and a holder for the at leastpartially-transparent sample, an optical metrology system for measuringthe location of the auxiliary reference surface as well as the nominallyparallel upper and lower surface areas of the sample, and a dataprocessing system for evaluating the optical and dimensional propertiesof the sample using the information derived from the calculated opticaland physical thickness at two or more positions of the nominallyparallel upper and lower surface areas of the sample.

In some embodiments, a complete measurement cycle includes a separatemeasurement of the topography of the auxiliary reference surface.

In some embodiments, the apparatus includes element(s) to alter thepolarization state of the light employed by the optical metrology system(e.g., a polarizer and/or wave plate), so as to evaluate and/orcompensate for material birefringence properties of the sample.

In some embodiments, the field of view of the optical metrology deviceextends beyond the lateral extent of the optical component under test.For improved accuracy and reduced sensitivity to drift, the systemperforms an additional measurement of the location of the referencesurface with respect to its internal datum, in areas where the referencesurface is not covered by the optical component.

In some embodiments, the optical metrology device is a coherencescanning interferometer. For example, the optical metrology device canbe a coherence scanning interferometer that is scanned nominallyparallel to an optical axis of the optical component under test. Thecoherence properties of the light source may be chosen to enhance (e.g.,maximize) the signal to noise ratio of the interference signal collectedfor each transparent interface encountered during scan through thecomponent.

In some embodiments, the coherence scanning interferometer automaticallyadjusts the light source coherence properties to enhance (e.g.,maximize) signal to noise ratio according to measured or nominalinformation about the part thickness and optical properties.

In some embodiments, measurements performed with the sample at multipleazimuthal orientations with respect to the instrument are combined toproduce a final result with reduced systematic error.

In some embodiments, a confocal microscope configuration is used fordetecting the location of the transparent interfaces of the sample byscanning.

In certain embodiments, a focus sensing or structured illuminationmetrology device is used for detecting the location of the transparentinterfaces of the sample by scanning.

In certain embodiments, a focus-sensing or structured-illuminationmetrology device is used for detecting the location of the transparentinterfaces by scanning.

In some embodiments, the optical radiation used for the metrology ischosen in the ultraviolet, visible or infrared spectra. Measurements arepreferably performed within a spectral domain close to that for whichthe optical component is designed.

In certain implementations, the apparatus and method may be used tocharacterize the thickness and refractive index of molded lenses.Alternatively, or in addition, the methods and apparatus may be used tocharacterize the thickness and lateral distances between features ofmolded lenses (e.g., critical features).

In general, typical lens molding processes rely on the accuratealignment of two molds facing one another. The distance between themolds defines the thickness of the molded components. Lens thickness isa critical parameter for the performance of the final lens assembly. Thedisclosed methods and apparatus can provide process control informationfor apex-to-apex thickness, e.g., with sub-micrometer accuracy.

Thickness variations across a lens provide quantitative informationabout the relative tilt of the two halves of the lens, another parameterthat is often critical for the final lens imaging capability. The tiltor parallelism error is thus another process control parameter that maybe measured by the disclosed apparatus and methods.

The refractive index and its variations within the lens also providerelevant information for process control. Out-of-tolerance refractiveindex variations or stress birefringence indicate problems with theinjection molding process. Both parameters affect the imagingperformance of the optical component. Both parameters may bequantitatively assessed with the disclosed apparatus and method forprocess control.

The lateral centering between the molds defines the apex-to-apexcentering of the lens, another critical parameter for the final lenscapability. The disclosed methods and apparatus can provide processcontrol information for apex-to-apex centering, e.g., withsub-tenth-micrometer precision.

Molding process parameters also influence the fill factor within themold and hence apex height and centering relative to locating features.Out-of-tolerance apex-to-feature height and apex-to-feature centeringcan indicate problems with the injection molding process. Bothparameters affect the imaging performance of the optical component. Bothparameters are quantitatively assessed with the disclosed methods andapparatus for process control.

Various aspects of the invention are summarized as follows.

In general, in a first aspect, the invention features a method fordetermining information about a transparent optical element composed ofan active portion (e.g., a lens portion) and an inactive portion (e.g.,a plane parallel portion), the active portion including at least onecurved surface and the inactive portion including opposing first andsecond surfaces, the method including:

directing measurement light to the transparent optical element;detecting measurement light reflected from at least one location on thefirst surface of the inactive portion; detecting measurement lightreflected from the second surface of the inactive portion at a locationcorresponding to the at least one location on the first surface;determining, based on the detected light, information about the inactiveportion; and evaluating the transparent optical element based on theinformation about the inactive portion.

Implementations of the method may include one or more of the followingfeatures and/or features of other aspects. For example, the surfacemeasurements of the first and second surfaces of the inactive portioncan be performed using coherence scanning interferometry (CSI).

Alternatively, the surface measurements of the first and second surfacesof the inactive portion are performed using confocal microscopy.

The information about the inactive portion can include a height profileof the first surface of the inactive portion and a height profile of thesecond surface of the inactive portion. The information about theinactive portion can include a physical thickness profile or opticalthickness profile of the inactive portion.

The information about the inactive portion can include information abouta refractive index of a material forming the transparent opticalelement. For example, the information about the refractive index caninclude a group index of the material and/or a phase index of thematerial. The information about the refractive index can includeinformation about variations of a refractive index between differentlocations of the inactive portion. The information about the refractiveindex can include information about birefringence of the materialforming the transparent optical element (e.g., plastic).

In some embodiments, the method further includes detecting measurementlight reflected from a reference feature on a fixture that supports thetransparent optical element and determining, based on the detected lightfrom the reference feature information about the reference feature. Themeasurement light reflected from the reference feature may be reflectedfrom a location corresponding to the at least one location on the firstsurface of the inactive portion (e.g., imaged to the same location ofthe detector). The measurement light can be transmitted by thetransparent optical element before and after being reflected from thereference feature. In some embodiments, the transparent optical elementis not in the path of the measurement light reflected from the referencefeature. In some cases, the method can include detecting measurementlight reflected from the fixture at a second location different fromlocation corresponding to the at least one location of the first surfaceof the inactive portion.

The measurement light can be detected for a first polarization and,thereafter, a second polarization different from the first polarization.

Evaluating the transparent optical element can include inferringinformation about a dimensional or optical property of the activeportion based on the information about the inactive portion.

In some embodiments, the inactive portion is a tilt control interlock ofthe transparent optical element. The at least one curved surface of theactive portion can be a spherical surface or an aspheric surface. Theactive portion can include a second curved surface opposite the firstcurved surface.

The information about the active portion can include information aboutbirefringence of a material forming the lens portion. The informationabout the active portion can include information about variations of arefractive index of a material forming the lens portion.

Evaluating the transparent optical element can include determiningwhether the transparent optical element meets a specificationrequirement based on the information about the inactive portion. Theinactive portion can be located around a circumference of the activeportion.

In a further aspect, the invention features a method of forming anoptical assembly, including:

determining information about the transparent optical element using theforegoing method, where the transparent optical element is a lens; andsecuring the lens relative to one or more other lenses in a barrel toform the optical assembly. The method can include securing the opticalassembly relative to a sensor to provide a module for a digital camera.

In general, in another aspect, the invention features a system fordetermining information about a transparent optical element thatincludes an active portion (e.g., a lens portion) and an inactiveportion (e.g., plane parallel portion), the active portion including atleast one curved surface and the inactive portion including opposingfirst and second surfaces, the system including: a fixture forsupporting the transparent optical element; an optical instrumentincluding a light source, a detector, and optical elements arranged todirect light from the light source towards the transparent opticalelement when the transparent optical element is supported by the fixtureand direct light reflected from the transparent optical element to thedetector; and an electronic controller in communication with thedetector, the electronic controller being programmed to determineinformation about the inactive portion based on light detected fromcorresponding locations of the first and second surfaces of the inactiveportion.

Embodiments of the system can include one or more of the followingfeatures and/or features of other aspects. For example, the opticalinstrument can be an optical areal surface topography instrument, suchas a coherence scanning interferometer or a confocal microscope.

The fixture can include a reference feature located in a path of thelight from the optical instrument. In some embodiments, the referencefeature is a planar reflector. The fixture can include a stand whichpositions the transparent optical element a distance from the referencefeature. The fixture may include an actuator for rotating thetransparent optical element relative to an optical axis of the opticalinstrument.

The light source may be capable of providing light having variablespectral content. The optical instrument can include a polarizationmodule configured to polarize light from the light source. Thepolarization module may be configured to selectively polarize light fromthe light source in orthogonal polarization states.

In general, in a further aspect, the invention features a method fordetermining information about a transparent optical element having alens portion and a plane parallel portion, the method including: usingan optical instrument to obtain height information (e.g., a surfaceprofile) about a first surface of the transparent optical element and asecond surface of the transparent optical element opposite the firstsurface; using the optical instrument to obtain an intensity map (e.g.,an image) of the first surface and an intensity map of the secondsurface; and determining, based on the height information and theintensity maps, dimensional information about one or more features ofthe transparent optical element on at least one of the first surface andthe second surface.

Implementations of the method may include one or more of the followingfeatures and/or features of other aspects. For example, the opticalinstrument can be a coherence scanning interference microscope or aconfocal microscope.

The height information about the first and second surfaces can includesurface profiles of the first and second surfaces, respectively. Themethod of claim 1, wherein the intensity map is determined based on asequence of intensity frames collected using a multi-element detector ofthe optical instrument. The intensity map is determined by averaging theintensity at each element of the multi-element detector for the sequenceof intensity frames. Using the optical instrument to obtain theintensity maps can include determining an intensity for each element ofthe multi-element detector at respective positions of best focus for thefirst and second surfaces relative to the optical instrument.

The dimensional information may include the location of an apex of thefirst or second surface relative to another feature on the first orsecond surface. In some cases, the dimensional information is a lateraldistance between the apex and the other feature, the lateral distancebeing a distance measured in a plane nominally parallel to the planeparallel portion. The other feature can be a feature located at theplane parallel portion of the first or second surface. The other featurecan be an annular feature, nominally centered on the apex. The otherfeature can be a step in the first and/or second surface of the planeparallel portion.

The optical instrument may be used to perform a measurement of thetransparent optical object with the first surface facing the opticalinstrument and to perform a measurement of the transparent opticalobject with the second surface facing the optical instrument. Dataacquired from the measurement of the transparent optical object with thefirst surface facing the optical instrument may be used to determine alocation of an apex of the lens portion of the first surface. Dataacquired from the measurement of the transparent optical object with thefirst surface facing the optical instrument may be used to determine thelocation of the apex of the lens portion of the first surface relativeto a location of a feature on the plane parallel portion of the firstsurface. Data acquired from the measurement of the transparent opticalobject with the first surface facing the optical instrument may be usedto determine a location of a feature on the plane parallel portion ofthe first surface relative to a location of a feature on the planeparallel portion of the second surface. Data acquired from themeasurement of the transparent optical object with the second surfacefacing the optical instrument may be used to determine a location of anapex on the lens portion of the second surface relative to the locationof the apex on the first surface.

Determining the dimensional information can include accounting for aneffect of refraction due to a tilt of the transparent optical elementrelative to the optical instrument. The dimensional information forwhich the effect of refraction is accounted can be a location of afeature on the surface of the transparent optical element opposite theoptical instrument.

The optical instrument can be used to perform a measurement of thetransparent optical object with a first azimuthal orientation relativeto an axis of the optical instrument and to perform a measurement of thetransparent optical object with the second azimuthal orientationrelative to the axis different from the first azimuthal orientation.Determining the dimensional information can include determiningdimensional information about the one or more features from dataobtained from the measurement with the transparent optical element withthe first azimuthal orientation and determining dimensional informationabout the one or more features from data obtained from the measurementwith the transparent optical element with the second azimuthalorientation. Determining the dimensional information can includereducing error in the dimensional information based on the dimensionalinformation obtained for the first and second azimuthal orientations.

The method can include determining whether the transparent opticalelement meets a specification requirement based on the dimensionalinformation.

In another aspect, the invention features a system for determininginformation about a transparent optical element, including: an opticalinstrument, and an electronic controller in communication with theoptical instrument and programmed to cause the system to perform themethod of the prior aspect.

Embodiments of the system may include one or more features of otheraspects.

In general, in another aspect, the invention features a method fordetermining information about an object including a curved portion and aplanar portion, the curved portion including a first curved surfacehaving an apex and defining an axis of the object, the method including:directing measurement light to the object; detecting measurement lightreflected from the first curved surface of the curved portion; detectingmeasurement light reflected from at least one other surface of theobject; and determining, based on the detected light, information aboutthe apex of the first curved surface of the curved portion.

Implementations of the method may include one or more of the followingfeatures and/or features of other aspects. For example, the object canbe a transparent optical element, such as a lens element (e.g., a moldedlens element). In some embodiments, the object is portion of a mold foran optical element, such as a mold for one side of a lens element.

The curved portion can include a second curved surface opposite thefirst curved surface, the second curved surface having an apex, and theinformation about the apex of the first curved surface includes athickness of the lens between the apex of the first surface and the apexof the second surface measured along the optical axis.

The curved portion can include a second curved surface opposite thefirst curved surface, the second curved surface having an apex, and theinformation about the apex of the first curved surface includes alateral offset between the apex of the first surface and the apex of thesecond surface measured in a plane orthogonal to the optical axis.

The measurement light can be directed to the object by an opticalinstrument and the first curved surface faces the optical instrumentwhen reflecting the measurement light. Determining the information aboutthe apex of the first curved surface can include determining a locationof the apex. The at least one other surface can include another surfacefacing the optical instrument and determining the information about theapex of the first curved surface further can include determining alateral offset measured in a plane orthogonal to the optical axisbetween the apex and a feature of interest on the at least one othersurface. The at least one other surface can include a surface facingaway from the optical instrument and determining the information aboutthe apex of the first curved surface further can include determining alateral offset measured in a plane orthogonal to the optical axisbetween a feature on the surface facing away from the optical instrumentand the feature of interest on the other surface facing the opticalinstrument. The curved portion can include a second curved surfaceopposite the first curved surface and determining the information aboutthe apex of the first curved surface includes determining a location ofthe apex of the second curved surface. Determining the information aboutthe apex of the first curved surface can include determining a thicknessof the curved portion measured along the optical axis based on thelocations of the first and second curved surfaces apexes. In someembodiments, determining the information about the apex of the firstcurved surface includes determining a lateral offset between the apex ofthe first surface and the apex of the second surface measured in a planeorthogonal to the optical axis based on: (i) the lateral offset betweenthe apex of the first curved surface and the feature of interest on theother surface facing the optical instrument; (ii) the lateral offsetbetween the feature of interest on the other surface facing the opticalinstrument and the feature of interest on the surface facing away fromthe optical instrument; and (iii) the lateral offset between the apex ofthe second curved surface and the feature of interest on the surfacefacing away from the optical instrument.

Determining information about the apex of the first curved surface caninclude determining information about a tilt of at least one surface ofthe planar portion and accounting for the tilt when determining theinformation about the apex of the first surface. The information aboutthe tilt is a tilt angle, α_(tilt), relative to an optical axis of anoptical instrument used to direct the measurement light to the object.

The method may include adjusting an azimuthal orientation of the objectwith respect to an optical instrument used to direct the measurementlight to the object after detecting the measurement light, and repeatingthe detection of measurement light from the first curved surface andfrom the at least one other surface after the azimuthal orientationadjustment. The method may include determining additional informationabout the apex of the first curved surface based on the detectedmeasurement light after the azimuthal orientation adjustment.

In some embodiments, the method includes changing a polarization stateof the measurement light after detecting the measurement light, andrepeating the detection of measurement light from the first curvedsurface and from the at least one other surface after the polarizationstate change. The method can include determining information about abirefringence of the object based on the detected measurement lightbefore and after the polarization state change.

The method may include evaluating the object based on the informationabout the apex of the first curved surface. Evaluating the object caninclude determining whether the object meets a specification requirementbased on the information about the apex of the first curved surface.

The planar portion can be a tilt control interlock of the object. The atleast one curved surface of the curved portion can be an asphericsurface. The planar portion can be located around a circumference of thecurved portion.

In a further aspect, the invention features a method of forming anoptical assembly, including: determining information about the objectusing the foregoing method where the object is a lens; and securing thelens relative to one or more other lenses in a barrel to form theoptical assembly. The method may include securing the optical assemblyrelative to a sensor to provide a module for a digital camera.

In a further aspect, the invention features a system for determininginformation about an object including a curved portion and a planarportion, the curved portion having a first curved surface having an apexand defining an axis of the object, the system including: a fixture forsupporting the object; an optical instrument including a light source, adetector, and optical elements arranged to direct light from the lightsource towards the object when the object is supported by the fixtureand direct light reflected from the object to the detector; and anelectronic controller in communication with the detector, the electroniccontroller being programmed to determine information about the apex ofthe first surface based on light detected from the first curved surfaceand from at least one other surface of the object.

Embodiments of the system may include one or more of the followingfeatures and/or features of other aspects. For example, the opticalinstrument can be an optical areal surface topography instrument, suchas a coherence scanning interferometer or a confocal microscope.

The fixture can include an actuator configured to reorient the objectwith respect to the optical instrument. For example, the actuator can beconfigured to rotate the object relative to an optical axis of theoptical instrument.

The optical instrument can include a polarization module configured topolarize light from the light source. The polarization module can beconfigured to selectively polarize light from the light source inorthogonal polarization states (e.g., using one or more polarizersand/or waveplates).

The detector can be a multi-element detector (e.g., a CMOS array or aCCD array) and the optical instrument can be configured to image asurface of the object onto the multi-element detector.

The light source can be capable of varying its spectral output. Forexample, the light source can include two or more LEDs of differingcolor. Varying the relative light intensity from the two or more LEDsvaries the color of the light. The light source can be a visible and/orinfrared light source.

Other aspects and advantages of the invention will be apparent from thedescription below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view an imaging optical lens assembly.

FIG. 2A is a side view of a sample lens under test.

FIG. 2B is a top view of the sample lens under test shown in FIG. 2A.

FIG. 3 is a schematic diagram of a coherence scanning interferometrymicroscope.

FIG. 4A is a side view of the sample lens under test mounted on afixture having a reference surface.

FIGS. 4B and 4C are side views of the sample lens and fixture shown inFIG. 4A, showing two steps in a measurement sequence.

FIG. 5 is a flowchart showing a process flow for an implementation usingcoherence scanning interferometry (CSI) and the steps shown in FIGS. 4Band 4C.

FIGS. 6A and 6B are side views showing measurement steps for anotherimplementation.

FIG. 7 is a flowchart showing a process flow for an implementation usingCSI and the steps shown in FIGS. 6A and 6B.

FIG. 8 is a flowchart showing a process flow for an implementationmeasuring birefringence between the upper and lower parallel surfaces ofa lens.

FIG. 9 is a side view of another sample lens under test.

FIG. 10 shows is a top view of the sample lens under test shown in FIG.9.

FIG. 11 shows another top view of the sample lens under test shown inFIG. 9, depicting in more detail lateral locations of interest withexaggerated decentering for illustration purposes.

FIG. 12 is a schematic diagram showing apparent and actual best focusposition for a feature of interest measured through the sample.

FIG. 13 shows a side view of the sample lens under test shown in FIG. 9,here with the concave lens surface facing the optical instrument.

FIG. 14 is a flowchart showing a process flow for an implementationusing CSI.

FIG. 15A is a schematic diagram showing apparent and actual position ofinterest corrected for refraction effects.

FIG. 15B illustrates azimuthal orientation of tilt and lateral shift dueto refraction.

FIG. 16 is a flowchart showing a process flow for another implementationusing CSI.

FIGS. 17A and 17B show top views of a lens in differentsample-to-instrument azimuthal orientations.

FIG. 18 is a flowchart showing process flow for a further implementationusing CSI.

FIG. 19 is a side view of a lens mold under test.

FIG. 20A is a flowchart showing a process flow for the characterizationof a lens.

FIG. 20B is a flowchart showing another process flow for thecharacterization of a lens.

FIGS. 21A and 21B are flowcharts showing process flows forcharacterizing a lens molding process.

DETAILED DESCRIPTION

Referring to FIGS. 2A and 2B, a lens 200 that includes an inactive,plane parallel portion 210 and an active, lens portion 220. In thisinstance, plane parallel portion includes two nominally planar,nominally parallel surfaces 211 and 212. Here, “nominal” refers to thedesign of the lens. Detectable deviations from nominal planarity ornominal parallelism may occur, e.g., due to manufacturing errors. Lensportion 220 is a meniscus lens, having a convex upper surface 221 and aconcave lower surface 222. In general, surfaces 221 and 222 may bespherical or aspherical. The plane parallel surfaces 211 and 212 may,for example, be features formed on the sample to aid in the alignmentand fixturing of the lens relative to one or more other lenses in afinal assembly.

An optical metrology instrument 201 is used to evaluate some of theoptical properties of the lens 200, including in particular therefractive index uniformity and residual stress birefringence, as wellas dimensional features such as the thickness of the lens, including butnot limited to the thickness, T, in the figure as a function of thecoordinates x, y (see the Cartesian coordinate system shown in FIG. 2A).The disclosed method performs these evaluations by measuring the opticalproperties and physical dimensions of the area between the planeparallel surface surfaces 211 and 212. These measurements serve asindicators of the overall optical and dimensional properties of thelens.

In general, optical metrology instrument 201 can be one of a variety ofdifferent instruments capable of performing an areal surface topographymeasurement of lens 200. Example instruments include coherence scanninginterferometry (CSI) microscopes (such as disclosed, e.g., in P. deGroot, “Coherence Scanning Interferometry,” in Optical Measurement ofSurface Topography, edited by R. Leach, chapt. 9, pp. 187-208, (SpringerVerlag, Berlin, 2011)), imaging confocal microscopes (such as disclosed,e.g., in R. Artigas, “Imaging Confocal Microscopy,” in OpticalMeasurement of Surface Topography, edited by R. Leach, chapt. 11, pp.237-286, (Springer Berlin Heidelberg, 2011)), structured illuminationmicroscopes (such as disclosed, e.g., in X. M. Colonna de Lega“Non-contact surface characterization using modulated illumination”,U.S. Patent (2014).), focus sensing (such as disclosed, e.g., in F.Helmli, “Focus Variation Instruments,” in Optical Measurement of SurfaceTopography, edited by R. Leach, chapt. 7, pp. 131-166, (Springer BerlinHeidelberg, 2011)) or wavelength tuned Fourier transform phase shiftinginterferometry (FTPSI) systems (such as disclosed, e.g., in L. L. Deck,“Fourier-Transform Phase-Shifting Interferometry,” Applied Optics 42(13), 2354-2365 (2003)).

Referring to FIG. 3, as an example, one type of optical metrology toolsuitable for characterizing lens 200 is a CSI microscope 300. In thissystem, a light source 302 directs input light 304 to an interferenceobjective 306 via relay optics 308 and 310 and beam splitter 312. Therelay optics 308 and 310 image input light 304 from spatially extendedsource 302 to an aperture stop 315 and corresponding pupil plane 314 ofthe interference objective 306 (as shown by the dotted marginal rays 316and solid chief rays 317).

In the embodiment of the FIG. 3, interference objective 306 is of theMirau-type, including an objective lens 318, beam splitter 320, andreference surface 322. Beam splitter 320 separates input light 304 intotest light 322, which is directed to lens 200 supported by a stage 326,and reference light 228, which reflects from reference surface 322.Objective lens 318 focuses the test and reference light to the test andreference surfaces, respectively. The reference optic 330 supportingreference surface 322 is coated to be reflective only for the focusedreference light, so that the majority of the input light passes throughthe reference optic before being split by beam splitter 320.

After reflecting from the test and reference surfaces, the test andreference light are recombined by beams splitter 320 to form combinedlight 332, which is transmitted by beam splitter 312 and relay lens 336to form an optical interference pattern on an electronic detector 334(for example, a multi-element CCD or CMOS detector). The intensityprofile of the optical interference pattern across the detector ismeasured by different elements of the detector and stored in anelectronic processor 301 (e.g., a standalone or networked computer, orprocessor integrated with other components of the system) for analysis.Relay lens 136 images different points in a focal plane of the objective306 to corresponding points on detector 134.

A field stop 338 positioned between relay optics 308 and 310 defines thearea of test surface 124 illuminated by test light 122. After reflectionfrom the lens 200 and reference surface, combined light 332 forms asecondary image of the source at pupil plane 314 of the objective lens.

Optionally, polarization elements 340, 342, 344, and 346 define thepolarization state of the test and reference light being directed to therespective test and reference surfaces, and that of the combined lightbeing directed to the detector. Depending on the embodiment, eachpolarization element can be a polarizer (e.g., a linear polarizer), aretardation plate (e.g., a half or quarter wave plate), or a similaroptic that affects the polarization state of an incident beam.Furthermore, in some embodiments one or more of the polarizationelements can be absent. Moreover, depending on the embodiment, beamsplitter 312 can be a polarizing beam splitter or a non-polarizing beamsplitter. In general, because of the presence of polarization elements340, 342 and/or 346, the state of polarization of test light 322 at testsurface 324 can be function of the azimuthal position of the light inpupil plane 314.

In the presently described embodiment, source 302 provides illuminationover a broad band of wavelengths (e.g., an emission spectrum having afull-width, half-maximum of more than 20 nm, of more than 50 nm, orpreferably, even more than 100 nm). For example, source 302 can be awhite light emitting diode (LED), a filament of a halogen bulb, an arclamp such as a Xenon are lamp or a so-called supercontinuum source thatuses non-linear effects in optical materials to generate very broadsource spectra (>200 nm). The broad band of wavelengths corresponds to alimited coherence length. A translation stage 350 adjusts the relativeoptic path length between the test and reference light to produce anoptical interference signal at each of the detector elements. Forexample, in the embodiment of FIG. 3, translation stage 350 is apiezoelectric transducer coupled to interference objective 306 to adjustthe distance between the test surface and the interference objective,and thereby vary the relative optical path length between the test andreference light at the detector.

Referring back to FIG. 2A, optical instrument 201 looks down at the lens200 along an observation direction parallel to the z-axis shown in FIG.2A. In the figure, S1 and S2 denote light reflections from the upper andlower nominally plane-parallel surfaces 211 and 212 on the lens 200,respectively. During metrology data acquisition the system collectsheight information of these surfaces in the global coordinate system x,y, z. This coordinate system is established by optical instrument 201.Ideally, the rotation axes of the lens surfaces is aligned nominallyparallel to the z-axis.

Metrology information for the upper surface 211 of the lens 200 isderived from the reflection of light in air (signal “S1” in the figure).Respectively, metrology information for the lower surface 212 of thelens 200 is derived from the reflection of light within the lensmaterial (signal “S2”) in the figure.

Considering the specific example of a CSI microscope system such assystem 300, the relative distance T between the upper and lower surfaces211 and 212 at a specific coordinate x, y will be given by

T=T′/n _(G)  (1)

where T′ is the apparent or measured optical thickness as determined byCSI microscopy or by wavelength-tuned FTPSI using coherence information,and n_(G) at low NA (e.g., 0.06 or less) is the group-velocity index ofrefraction (at high NA, e.g., 0.2 or more, the value n_(G) could changebecause of the obliquity effect, resulting in an effectivegroup-velocity index of refraction). Conversely, signal S2 will appearto originate at a higher z location when using confocal, structuredillumination or focus sensing. The physical thickness in this case isgiven by

T=nT″  (2)

where T″ is the apparent or measured optical thickness as determined byconfocal or related focus-sensitive instruments, and n is thephase-velocity index of refraction.

The thickness map T′(x,y) or T″(x,y) provides information about the meanvalue and uniformity of the physical thickness T(x,y) as well as theoptical properties of the lens 200 as exemplified by the index ofrefraction n_(G) (x,y) or n(x,y). In some cases, the compositeuniformity and mean value of both of these properties, dimensional andoptical, is sufficient for process control in the manufacture of thelens 200.

If desired, additional information such as the thickness map T(x,y) orthe optical refractive index n(x,y) obtained by other means, such as bycontact profilometry (as disclosed, e.g., in P. Scott, “RecentDevelopments in the Measurement of Aspheric Surfaces by Contact StylusInstrumentation,” Proc. SPIE 4927, 199-207 (2002)), may supplement themeasurements performed by the optical metrology instrument 201, allowingfor separation and independent evaluation of the effects of therefractive index from the physical thickness.

While the foregoing lens characterization relies on height profileinformation about surfaces 211 and 212 alone, lens characterization mayutilize other information too. For example, in some implementations, aspecialized reference fixture is included to provide additional opticalinformation. Referring to FIG. 4A, in certain embodiments, lens 200 ismounted on a fixture 400 having an embedded reference surface 420. InFIG. 4A, S1, S2 and S3 denote reflected light from the upper and lowersurfaces of interest (211 and 212) of the lens 200, and the uppersurface 420 of the fixture reference, which is separate from lower lenssurface 212 by a distance T_(air) as measured along the z-axis.

Fixture 400 includes support structures 410 and reflective upper surface420. Lens 200 rests on support structures 410, which position the lens adistance T_(air) from reflective surface 420. Support structures 410 maybe composed of multiple pillars or walls on opposing sides of lens 200,or may be a single cylindrical support separating an inner portion 422from an outer portion 421 of reflective surface 420. Fixture 400 may betailored specifically for lens 200, and may be replaced with anotherfixture when a different shaped lens is measured.

FIGS. 4B and 4C show two successive measurement steps with the opticalinstrument 201 measuring over the full field of the plane parallelportions 210 of the lens 200. These steps provide the metrologyinformation, including height profile measurements of surfaces 211, 212,and 420, to complete a characterization of the geometry and opticalproperties of the lens according to the flowchart 500 of FIG. 5.Apparent height measurements z_(1, . . . 4) correspond to reflectedlight S1 . . . S4, respectively, in the figure.

In a first step, depicted in FIG. 4B, metrology information is collectedfor the three surfaces 211, 212, and 420 (steps 510, 520, 530), wherethe lower surface 212 and auxiliary reference surface 420 are measuredthrough the lens material and hence correspond to apparent heights. Themetrology information is collected for the three surfaces withoutadjusting the position of the lens on the fixture.

In a second step, depicted in FIG. 4C, the lens is removed from thefixture 400 (step 540) and the auxiliary reference surface 420 ismeasured a second time (signal S4) (step 550).

The metrology information is combined to create maps of the thicknessand refractive index distribution between the upper and lower parallelsurfaces of the lens element. For coherence scanning interferometers andcomparable interferometric instruments, after acquiring the apparentheight information z_(1, . . . 4), the physical and optical thicknessmaps are, respectively:

T(x,y)=z ₁(x,y)−z ₂(x,y)+z ₃(x,y)−z ₄(x,y)  (3)

T′(x,y)=z ₁(x,y)−z ₂(x,y).  (4)

The map of the group-velocity refractive index is then

n _(G)(x,y)=T′(x,y)/T(x,y).  (5)

When the metrology system relies on confocal, structured illumination orfocus sensing surface profiling, Eqs. (4) and (5) become

T(x,y)=z ₁(x,y)−z ₂(x,y),  (6)

n=T/T″.  (7)

The thickness map provides information about the mean thickness of thelens as well as possible tilt between the two sides of the lens, basedon variations in the measured thickness from one side of the lens to theother. The refractive index map provides information about possiblerefractive index gradients across the lens area.

As an optional additional step, knowing the nominal refractivedispersion properties of the material in the lens, it is often possibleto transform the group index to the phase index:

n=Transform(n _(G)).  (8)

In some cases, the transform may be as simple as an additive constant.For instance, the additive constant is

${{- \frac{\partial{n(k)}}{\partial k}}k_{0}},$

where n(k) is the nominal refractive index of the material (as stated bythe manufacturer or measured through some other means), expressed as afunction of wavenumber, and k₀ is the centroid wavenumber of thespectral band used for the measurement. Other transforms are possiblesuch as a lookup table or polynomial function. Transform polynomials canbe created by fitting data points of measured group index values (usingthe instrument) as a function of the known refractive index of testsamples.

Additional measurements may also be made in order to improve theaccuracy of the process. For example, referring to FIGS. 6A, 6B, and 7,in some embodiments an additional measurement z₅(x,y) for a reflectionS5 from the fixture reference surface where the light path is unimpededby lens 200 is captured simultaneously (step 710 in flowchart 700) withthe lens image. This additional information provides, for example, theoverall height offset of the fixture with respect to the opticalinstrument, for the case where the fixture may move between the twomeasurement steps. This information can correct the results of thez₄(x,y) measurement by providing, for example, an offset or thecombination of an offset, tip and tilt compensation for changes in thefixture position between measurements (step 720).

In some embodiments, the measurement is repeated for differentconfigurations of the instrument such that data collection is performedwith substantially different spectral distributions, for example, afirst spectral distribution centered between 400 nm and 490 nm, a secondspectral distribution centered between 490 nm and 590 nm and a thirdspectral distribution centered between 590 nm and 700 nm. Each spectraldistribution provides an independent measurement of the opticalproperties of the lens material. The multiple measured values ofgroup-velocity index or phase-velocity index can then be combined toderive an estimate of the material optical properties variation withwavelength (or dispersion), which can be used to verify that thematerial is within tolerances and/or for controlling the manufacturingprocess. In the case where the instrument measures group-index (e.g. acoherence scanning interferometer), the estimate of dispersion isfurther used to compute an estimate of the refractive index, for exampleusing the product of the first order derivative by the centroidwavenumber. In some embodiments, the multiple spectral distributions arepresent concurrently while the instrument collects the data resultingfrom the scanning data acquisition. The multiple spectral bands areseparated at the detector, for example using a color sensitive device(CCD or CMOS camera equipped with color filters). Alternatively,returning light from the sensor is spatially separated by dichroicoptical elements that reflect or transmit specific spectral componentstoward multiple monochrome sensors. A minimum of two spectral bands isrequired to estimate the dispersion property of the material.

While the foregoing measurements may be performed using polarized orunpolarized light, it is possible to glean additional information aboutlens 200 using polarized light. For example, referring to FIG. 8,information about the polarization-dependent optical properties of thelens, including the effect of stress birefringence, which can revealproblems with the lens (or other partially-transparent sample)manufacture, may be determined. In most cases, a lens free of stress andassociated stress birefringence is a design goal for manufacturingprocess control.

The presence of stress birefringence in a sample may be monitored byobserving its effects in the plane-parallel areas of the sample. Here,the measurement process outlined in flowchart 500 or flowchart 700 isperformed at least twice, where each complete data acquisition cycle isperformed for a different polarization state of the illumination lightused by the metrology system. The polarization state of the opticalmeasurement instrument may be manipulated using convention polarizersand/or waveplates.

For example, as shown in flowchart 800, a first measurement is performedwith the illumination light linearly polarized along the x direction andrepeated with illumination light linearly polarized along theydirection. In some embodiments, the polarization directions are alignedwith respect to datum features on lens 200, such as where the lens is aninjection-molded lens the datum features may correspond to the gatewhere the injected material enters the mold cavity.

The multiple refractive index maps collected are then combined toprovide a quantitative measurement of birefringence present in the lensmaterial. For example, in step 870, a birefringence effect is calculatedfrom the measurements. In step 880, a mean refractive index iscalculated from the measurements. Birefringence may be, for example,expressed as the difference of optical paths through the lens, as shownin step 870 of flowchart 800. Here the cumulative effect ofbirefringence through the lens is calculated as

B(x,y)=[n ₂(x,y)−n ₁(x,y)]T  (9)

while the mean index (as shown in step 880) is

n(x,y)=0.5[n ₂(x,y)+n ₁(x,y)].  (10)

Birefringence can similarly be expressed as the difference of opticalpath per unit length of propagation within the material. Thephase-velocity refractive indices n_(1,2) correspond to the twopolarization orientations. For process control, these indices areadequately represented by the group index measurements that follow, forexample, from CSI microscope measurements. Further, for some processcontrol situations, a measurement of optical thickness variation

B′(x,y)=T′ ₂(x,y)−T′ ₁(x,y)  (11)

or

B″(x,y)=T″ ₂(x,y)−T″ ₁(x,y)  (12)

using the simpler configuration of FIG. 2A may already be sufficient.

While the foregoing embodiments involve measurements characterizing theinactive portion (e.g., plane parallel portion) of the lens andinferring information about the lens generally from thosecharacterizations, other implementations are also possible. For example,measurements directly characterizing the active portion of the lens canalso be performed.

Referring to FIGS. 9 and 10, a sample lens 900 under test, includes anactive portion 920 having curved surfaces, and an inactive portion 910,composed of several nominally planar surfaces surrounding the activeportion. FIG. 9 shows a side view of lens 900, while FIG. 10 shows a topview. The active portion 920 corresponds to a convex upper surface 921and a convex lower surface 922. Upper surface 221 has an apex 923nominally aligned along the same axis as an apex 924 of lower surface922.

The inactive portion 910 is composed of a series of planar, annularsurfaces with step features offsetting inner and outer planar surfaceson each side of lens 900. In general, the surfaces of inactive portion910 may, for example, include features formed on the sample to aid inthe alignment and fixturing of the lens in a final assembly, and/or tofacilitate measurement of the relative alignment of lens features. Inthis case, the upper side of inactive portion 910 includes planarsurfaces 912 and 916. A step 914 separates surfaces 912 and 916. Step914 meets surface 912 at edge 914 o and surface 916 at edge 914 i.Surface 916 meets upper convex surface 921 at edge 918.

The lower side of inactive portion 910 includes planar surfaces 911 and917. A step 915 separates surfaces 911 and 917. Step 915 meets surface911 at edge 915 o and surface 917 at edge 915 i. Surface 917 meets lowerconcave surface 922 at edge 919.

Optical metrology instrument 201 is used to evaluate some of thedimensional features of lens 900, including (but not limited) to theapex-to-apex thickness T_(Apex) and the relative x, y lateral offsets(referred to a common axis z) of surface feature locations, including(but not limited to) apex centers and alignment surface features. Theseevaluations are performed by measuring the upper surface profile todetermine 3D apex location and relative 3D location and topography ofother surface features. These measurements serve as indicators of theoverall dimensional properties of the lens.

During operation, optical instrument 201 looks down at the sample alongan observation direction parallel to the z-axis shown in FIG. 9,corresponding to an optical axis of instrument 201. During metrologydata acquisition the system collects height and intensity informationfor surfaces of interest in the global coordinate system x, y, z.

Metrology information for apex 923 is derived from the reflection oflight in air (signal S_(UA) in FIG. 9), as is metrology information forother features of interest in the upper surface (signal S_(UF) in FIG.9). Respectively, metrology information for features of interest in thelower surface is derived from the reflection of light within the lensmaterial (signal S_(LF) in FIG. 9).

Considering the specific example of a CSI microscope system such as thatshown in FIG. 3, signal S_(UA) is generally processed to produce heightinformation which can then be analyzed to determine the 3D location ofthe apex 923, P_(UA). Height information derived from signal S_(UF) canbe combined with P_(UA) to determine H_(UA), the apex height in zrelative to the upper surface feature of interest, surface 912. Thissame height information may also be used to determine the location ofupper surface edge features, for example nominally circular edges of theupper surface feature of interest 912. Alternatively, or additionally,signal S_(UF) can be processed to produce intensity information whichcan then be analyzed to determine the location of upper surface edgefeatures 914.

S_(LF) is a non-interferometric intensity signal which may be analyzedto determine the location of lower surface edge features 915. Referringto FIG. 12, S_(LF) may be measured with a microscope system at a zposition of best focus, shown in FIG. 12, displaced by T_(BF) relativeto the focal plane of the upper surface at that x, y location. Forthickness T_(feature) and refractive index n, T_(BF) can be computed fornear-normal incident angles as:

T _(BF) −T _(feature) /n  (13)

For this computation, thickness and index may be assumed nominal valuesor previously measured by some other means, e.g., using the sameinstrument or a caliper. Depending on the required accuracy for a givenapplication, it can further be beneficial to compensate for the effectof spherical aberration induced by refraction through the lens material,and compute a corrected value for T_(BF), e.g., using the formula:

$\begin{matrix}{{T_{BF} = {\frac{T_{feature}}{n}\left( {1 - {\frac{{NA}^{2}}{4}\left( {1 - \frac{1}{n^{2}}} \right)}} \right)}},} & \left( {13B} \right)\end{matrix}$

where NA refers to the numerical aperture of the optical instrument.

The lateral location of the upper surface apex C_(UA) is given by the x,y ordinates of P_(UA). The location of other features of interest can bedefined in other ways, for example as the center of measured edgepositions, indicated as C_(UF) and C_(LF) in FIG. 11. Lateral distancesbetween these locations correspond to offsets between axes parallel withthe z-axis, implicitly referred to a datum plane, and in some casescorresponding to a planar feature of the upper surface. For example, theinter-feature lateral distance XY_(Feature) can be computed as:

XY _(Feature) =C _(UF) −C _(LF)  (14)

Similarly, the upper surface apex-to-feature lateral distance XY_(UAF)can be computed as:

XY _(UAF) =C _(UA) −C _(UF)  (15)

In some cases, XY_(Feature) is sufficient for process control in themanufacture of the lenses, for example as a measure of the lateralalignment of the mold halves. Similarly, XY_(UAF) along with relativeapex height H_(UA) may be sufficient for identifying issues with lensformation, for example if these deviate from dimensions expected fromthe upper surface mold half.

It may be desired to explicitly measure dimensional properties betweenthe upper surface apex and the lower surface apex, such as apexthickness T_(Apex) indicated in FIG. 9 or inter-apex lateral distanceXY_(Apex), corresponding to the lateral distance between C_(UA) andC_(LA) indicated in FIG. 11. In some embodiments, and with reference toFIG. 13, this can be achieved by additionally measuring lens 900oriented with lower surfaces 911, 917, and 922 facing the opticalinstrument 201, while keeping track of azimuthal orientation relative tothe measurement made with the upper surfaces of lens 900 facing opticalinstrument 201. Using methods similar to those described for the firstmeasurement, this second measurement provides H_(LA), P_(LA), and lowersurface apex-to-feature lateral distance XY_(LAF), corresponding to thelateral distance between C_(LA) and C_(LF):

XY _(LAF) =C _(LA) −C _(LF)  (16)

Note that H_(LA) is negative for the particular geometry depicted inFIG. 13.

In some cases this second measurement can provide an independentmeasurement of XY_(Feature).

In some embodiments, metrology information from measuring the lens firstwith one surface facing the instrument, and then the other, is combinedaccording to flowchart 1400 shown in FIG. 14 to create a measurement oftotal apex thickness and desired lateral distances. In thisimplementation, the sequence of steps is as follows: first, lens 900 ispositioned with the upper surface facing metrology instrument 201 (step1405). While in this configuration, metrology instrument 201 measures aheight profile at least in the region of the apex of the upper surfaceand computes a location of this upper apex (step 1410). With lens 900 inthe same position, instrument 201 measures a height profile andintensity profile for an upper feature of interest, such as edge 914 o(step 1415). In step 1420, the system then computes upper apex height,H_(UA), and upper apex-to-feature lateral distance, XY_(UAF) (e.g.,using equation (15)).

For measurement of lower surface features, metrology instrument 201 andlens 900 are moved relative to each other so that a lower feature ofinterest, such as edge 915 o, is at a best focus position (step 1425).This location may be determined using nominal or measured values ofT_(feature) and n. In this position, the instrument measures anintensity profile for the lower feature (step 1430). Using informationfrom the intensity profile, the system computes (in step 1435) aninter-feature lateral offset XY_(Feature).

Next, lens 900 is flipped and positioned with its lower surface facinginstrument 201 (step 1440). In this position, a height profile ismeasured in the region of lower apex 924, and a lower apex location,P_(LA) is computed (step 1445). The system then, in step 1450, measuresa height profile and an intensity profile for one or more features onthe lower surface (e.g., edge 915). With this measurement, the systemcompute a lower apex height, H_(LA), and a lower apex-to-feature lateraldistance XY_(LAF) (step 1455).

In step 1460, apex thickness T_(Apex) can be computed as:

T _(Apex) =H _(UA) +T _(feature) +H _(LA)  (17)

Finally, in step 1465, inter-apex lateral distance XY_(Apex) correspondsto the lateral distance between C_(UA) and C_(LA) and can computedaccording to the following, where superscripts indicate whetherparameters are obtained from the upper surface measurement or the lowersurface measurement:

XY _(Apex) =XY _(UAF) ^(upper)(XY _(Feature))^(upper) −XY _(LAF)^(lower)  (18)

If the lower-surface measurement provides an independent measurement ofthe inter-feature lateral distance XY_(Feature), the followingexpressions can optionally be used to potentially reduce statisticalvariability:

XY _(Feature)=0.5[XY _(Feature) ^(upper) +XY _(Feature) ^(lower)]  (19)

XY _(Apex) =XY _(UAF) ^(upper) +XY _(Feature) −XY _(LAF) ^(lower)  (20)

In some embodiments, as discussed previously with respect to FIGS.4A-4C, the apparatus may include a part fixture that includes anominally flat reflective surface, placed under the sample such that itreflects light that propagated through the sample back through sampleand toward the metrology instrument. Such implementations may improvethe contrast in the intensity images acquired using optical instrument201.

In certain embodiments, the information regarding x, y spatialvariations in areas including the features of interest may be exploitedto more accurately determine dimensional features. For example, thisinformation could include maps of refractive index n(x,y), thicknessT(x,y), and surface topography S_(UA)(x,y) and S_(UA)(x,y).

Referring to FIGS. 15A and 15B, the planar surface areas of a lens undertest appear to be parallel but in practice there may be deviation fromthis ideal. For example, best-fit planes through the upper and lowerfeatures of interest may deviate from parallel. This can producenon-parallel part tilt for the first measurement (upper surface facinginstrument) relative to the second measurement (lower surface facinginstrument), for example if part tilt is adjusted relative tonon-parallel features in the upper and lower surface respectively. Theserelative part tilts form a wedge angle W which can be derived fromthickness map T(x,y) and incorporated into computations of dimensionalfeatures. For example, apex thickness T_(Apex) can be expressed as:

T _(Apex) =f _(ApexZ)(H _(UA) ,T _(feature) ,H _(LA) ,W)  (21)

Lateral distances XY_(Feature) and XY_(Apex) can be expressed as:

XY _(Feature) =f _(FeatureXY)(C _(UF) ,C _(LF) ,W)  (22)

XY _(Apex) =f _(ApexXY)(XY _(UAF) ,XY _(Feature) ,XY _(LAF) ,W)  (23)

FIG. 9 depicts a lower surface edge measured through an upper surfaceinterface that appears to be perpendicular to the optical axis of theareal surface topography instrument but again in practice there may bedeviation from this ideal. Moreover, this deviation can have (x,y)dependence, for example manifesting as variations in local tilt in thesurface topography map S_(UA)(x,y). FIG. 15A depicts a position ofinterest at a particular lateral location measured through a thickness Tin a material with refractive index n measured by a light beam firstencountering a surface tilted by α_(tilt) from perpendicular.

Due to refractive effects, there will be a lateral shift ΔL between theapparent and actual lateral location of the position of interest givenapproximately by:

ΔL=T sin(α_(refr))  (24)

where sin(α_(refr)) and sin(α_(tilt)) are related via Snell's law:

sin(α_(refr))=sin(α_(tilt))/n.  (25)

Thus, ΔL is given by:

ΔL=T sin(α_(tilt))/n.  (26)

In FIG. 15A, thickness T is depicted as being measured along thedirection of the beam, as expected for certain thickness measurementmethods with the sample in the same orientation. For some embodiments, Tmay correspond to the thickness along the optical axis. For the smallvalues of α_(tilt) typically encountered the influence on ΔL of thispotential difference is negligible.

Local tilt α_(tilt) will have some azimuthal orientation θ_(tilt) in theXY plane. As shown in FIG. 15B, lateral shift ΔL will have the sameazimuthal orientation. Corrections to the x and y ordinates of theapparent location of the position of interest are then respectively:

Δx=ΔL·cos(θ_(tilt))  (27)

Δy=ΔL·sin(θ_(tilt))  (28)

In general, index n, thickness T, tilt α_(tilt) and azimuthalorientation θ_(tilt) will depend on lateral location (x,y), so ΔL willalso generally be a function of (x,y). Refraction correction can beapplied to each measured edge point, following which the collection ofcorrected edge points can be analyzed as desired to generate a correctedlocation for the feature of interest.

Referring to FIG. 16, an exemplary implementation that accounts for atilt angle of the lens is shown in flowchart 1600. Here, the lens isfirst positioned with its upper surface facing the metrology instrument(step 1605) and the instrument is used to measure the height profile ofthe upper apex region and, from this measurement, compute the upper apexlocation P_(UA) (step 1610). The system then measures a height andintensity profile for an upper feature of interest, such as uppersurface 915 or an edge on the upper surface (step 1615). From thismeasurement, the system computes an upper feature center, C_(UF). Instep 1620, the system then computes an upper apex height HUA and upperapex-to-feature lateral distance XY_(UAF). For the next measurement,using nominal or measured values for T_(feature) and n, the system movesthe lens relative to the optical instrument so that a lower feature ofinterest (e.g., a planar surface or edge) is in a best-focus position(step 1625). In this position, the system measures an intensity profilefor the lower surface feature, and measures apparent edge positions onthe lower surface (step 1630). These measurements are corrected by thesystem for lateral shift, ΔL, for each edge position using local valuesof index n, tilt α_(tilt), and thickness, T (step 1635). Using thecorrected edge positions, the system calculates a position of the centerof the lower feature, C_(LF) (step 1640). Having the positions of theupper and lower features (C_(UF) and C_(LF)), and a wedge angle, W, thesystem computes an inter-feature lateral offset, XY_(Feature) (step1645). Here, the wedge angle corresponds to tilt in a thickness map ofthe lens.

Next, in step 1650, the lens is flipped so that the lower surface facesthe optical instrument (step 1650) and a height profile of the lowerapex region is acquired (step 1655). The system computes the lower apexlocation, P_(LA), from this height profile. The system then measures aheight profile and an intensity profile for the lower surface feature ofinterest (step 1660). H_(LA), the lower surface apex height, andXY_(LAF), the lower apex-to-feature lateral distance, are then computed(step 1665) from the information acquired from steps 1655 and 1660.Using this value for H_(LA), along with values for H_(UA) andT_(feature), the system computes apex thickness, T_(Apex) (step 1670).Using XY_(UAF), XY_(LAF), XY_(Feature), and W, the system also computesa value for the inter-apex lateral offset, XY_(Apex) (step 1675).

In some embodiments, the sample is measured at two or more azimuthalorientations relative to the optical instrument. By obtainingindependent measurements of dimensional properties of the lens atdifferent azimuthal orientations, the system can combine theseindependent measurements so as to reduce systematic error in the finalreported dimensional properties.

Examples of sources of systematic error include misalignment between theoptical axis and the scan axis, lateral or axial misalignment of theillumination, and bias in sample tilt.

In some cases, systematic error has a component that is independent ofsample orientation. For example, the reported lateral distance betweentwo particular features may be biased by some offset in instrumentcoordinates (Δx_(bias), Δy_(bias)). This bias can depend on theparticular sample features being measured. In such cases, systematicerror in measured lateral distance can be reduced by combiningmeasurements with the sample at an azimuthal orientation θ₀ relative tothe instrument as well as with the sample at an azimuthal orientationθ₁₈₀ relative to the instrument, where θ₁₈₀ should be offset by 180°relative to θ₀. As depicted in FIGS. 17A and 17B, this corresponds to arelative azimuthal rotation of 180° between sample coordinates(x_(sample), y_(sample)) and instrument coordinates (x_(instr),y_(instr)). This relative azimuthal orientation can be achieved viasample fixturing or by aligning to a distinctive feature on the partitself. For instance, the sample support may include a rotation stageand scale that can be manually or automatically rotated about theoptical axis of the optical instrument by a desired amount.

Referring top FIG. 18, an exemplary method utilizing sample rotation isshown in flowchart 1800. This process combines measurement sequencesperformed with the lens in four distinct orientations relative to theinstrument:

-   -   Upper surface facing instrument at azimuthal orientation θ₀    -   Upper surface facing instrument at azimuthal orientation θ₁₈₀    -   Lower surface facing instrument at azimuthal orientation θ₀    -   Lower surface facing instrument at azimuthal orientation θ₁₈₀

Specific steps are as follows. First, the lens is positioned with itsupper surface facing the optical instrument and at azimuthal orientationθ₀ (step 1805). In this orientation, the system performs a sequence ofheight and intensity profile measurements and computes values forXY_(UAF) ⁰ and XY_(Feature) ⁰ (step 1810).

For the next measurement sequence, the lens is positioned with its uppersurface facing the optical instrument and at azimuthal orientation θ₁₈₀(step 1815). In this orientation, the system performs a sequence ofheight and intensity profile measurements and computes values forXY_(UAF) ¹⁸⁰ and XY_(Feature) ¹⁸⁰ (step 1820).

For the subsequent measurement sequence, the lens is positioned with itslower surface facing the optical instrument and at azimuthal orientationθ₀ (step 1825). In this orientation, the system performs a sequence ofheight and intensity profile measurements and computes a value forXY_(LAF) ⁰ (step 1830).

For the final measurement sequence, the lens is positioned with itslower surface facing the optical instrument and at azimuthal orientationθ₁₈₀ (step 1835). In this orientation, the system performs a sequence ofheight and intensity profile measurements and computes a value forXY_(LAF) ¹⁸⁰ (step 1840). The order in which measurements are made atthese relative orientations is not critical, and can be governed by whatis most convenient.

The computed values are next used to compute constituent inter-apexlateral distances for each orientation, XY_(Apex) ⁰ and XY_(Apex) ¹⁸⁰(step 1845). Finally, using these constituent values, the systemcomputes an inter-apex lateral distance, XY_(Apex) ^(final) (step 1850),and final apex-to-feature lateral distances, XY_(UAF) ^(final) andXY_(LAF) ^(final) (step 1855).

Final reported lateral distances XY^(final) are computed by combiningthe corresponding lateral distances measured at θ₀ and θ₁₈₀,respectively XY⁰ and XY¹⁸⁰:

XY ^(final) =f _(Combine)(XY ⁰ ,XY ¹⁸⁰)  (29)

The preceding equation can be applied to lateral distances of interest,including those discussed previously such as inter-feature lateraldistance XY_(Feature), apex-to-feature lateral distances XY_(UAF) andXY_(LAF), and inter-apex lateral distance XY_(Apex). If constituentmeasurements of lateral distances are all in the sample's frame ofreference, i.e., relative to sample coordinates (x_(sample),y_(sample)), in some cases the combining function may be as simple asthe arithmetic mean of constituent measurements. Alternatively, oradditionally, some operations may map tool-reference-frame results tosample-reference-frame in a single step. Possible other operations canaccount for previously determined remnant tool bias.

As noted previously, another potential source of measurement error ismaterial birefringence in the sample. In some cases, measurement errorcan be reduced by combining measurements obtained with the instrument ina variety of polarization states, for example using a polarizer and/orwaveplate. This can further be combined with variations in relativeazimuthal orientation of the sample relative to the instrument.

The apparatus and methods described above allow for evaluation ofin-process transparent samples, including in particular, lensesincluding curved active surface areas as well as plane-parallel areasthat serve as surrogates for determining the dimensional and opticalproperties of the samples. Transparent samples include lenses, such asmolded lenses that are part of multi-lens lens assemblies, e.g., fordigital cameras. Such lens assemblies are extensively used in camerasfor mobile devices, such as cell phones, smart phones, and tabletcomputers, among other examples.

In some embodiments, the foregoing methods may be applied to measuring amold for the lens. For example, referring to FIG. 19, the sample undertest may be a half of a lens mold 1900. This mold includes a curvedsurface 1921, corresponding to the active area of lenses formed usingthe mold. Curved surface 1921 has an apex 1921. The mold also includes aplanar portion composed of a first, inner planar surface 1916 and asecond, outer planar surface 1912. The planar surfaces are separated bya step 1914. The outer edge 1914 o of step 1914 may be used as a featureof interest in measurements characterizing mold 1900. Outer, planarsurface 1912 and apex 1923 are offset by a height HA measured along thez-axis of instrument 201. Mold 1900 may be characterized by acquiringheight profiles and intensity profiles from, for example, surface 1921(via light SA) and outer, planar surface 1912 (SF). Information about,e.g., apex location and a lateral offset of the apex to the edge of thefeature of interest may be determined as described previously for lens900.

The flowcharts in FIGS. 20A and 20B illustrate possible uses of thedescribed techniques. Flowchart 2000 in FIG. 19 shows a lenscharacterization technique that uses measurements of thickness andbirefringence (step 2010) as described above. In step 2020, based onthese measurements, the system reports values for thickness,parallelism, mean refractive index, index gradient, and birefringence.These values are next compared to predetermined specifications for theseparameters (step 2030).

For those lenses out of specification, the lenses are rejected (step2040) and the system reports the corresponding molding sites as beingoutside process control targets (step 2050). For those lenses that meetspecification, the lenses are sorted into thickness bins (step 2060) andthe corresponding molding sites are reported as being within processcontrol targets (step 2070).

Flowchart 2001 in FIG. 20B shows a lens characterization technique thatuses measurements of apex-to-feature height, apex thickness, inter-apexlateral distance, and inter-feature lateral distance. Here, in a firststep 2011, locations of various features on the lens is derived fromheight and intensity data measured as described above. In step 2021,based on these measurements, the system reports values forapex-to-feature height, apex thickness, inter-apex lateral distance, andinter-feature lateral distance. These values are next compared topredetermined specifications for these parameters (step 2031).

For those lenses out of specification, the lenses are rejected (step2041) and the system reports the corresponding molding sites as beingoutside process control targets (step 2051). For those lenses that meetspecification, the lenses are sorted into thickness bins (step 2061) andthe corresponding molding sites are reported as being within processcontrol targets (step 2071).

The measurement techniques can also be used to characterize the moldingprocess used to make lenses. Implementations for characterizing themolding process are shown in the flow charts in FIGS. 21A and 21B.

In the process shown in flowchart 2100 in FIG. 21A, the first step 2110is to measure the thickness and birefringence of the lens's inactiveportion. Based on these measurements, the system analyzes thickness,parallelism, mean refractive index, index gradient and birefringence ofthe lens (step 2120). Based on this analysis, a relative position of thetwo halves of the mold are adjusted to meet thickness and parallelismspecifications (step 2130). Molding process parameters (e.g.,temperature and temperature ramp rates, lens material composition,injection pressure) are adjusted in order to meet refractive indexspecifications (step 2140).

In the process shown in flowchart 2101 in FIG. 21B, the first step 2150is to measure the height profile of lens surfaces and lateral positionsof the apexes and features. Based on these measurements, the systemdetermines and reports apex-to-feature heights, apex thickness,inter-apex lateral distance, and inter-feature lateral distance (step2160). Based on this analysis, the relative position of the two halvesof the mold is adjusted to meet thickness and lateral centeringspecifications (step 2130). Molding process parameters (e.g.,temperature and temperature ramp rates, lens material composition) areadjusted in order to meet apex-to-feature specifications (step 2140).

Although the foregoing flowcharts depict separate processes forbirefringence and thickness measurements than for apex and featuremeasurements, in some embodiments both these sets of measurements may becombined in order to, for example, improve lens characterization and/orlens molding.

While certain implementations have been described, other implementationsare also possible. For example, while lens 200 and lens 900 are bothmeniscus lenses, more generally other types of lenses may becharacterized using the disclosed techniques including, for example,convex-convex lenses, concave-concave lenses, plano-convex lenses, andplano-concave lenses. Lens surfaces may be aspherical. In someembodiments, lens surfaces may include points of inflection where theconcavity of the surface changes. An example of such a surface issurface 132 in FIG. 1.

Moreover, a variety of alignment features in addition to thoseillustrated above may be used. For example, while the planar surfaces inlenses 200 and 900 are annular surfaces, other geometries are possible.Discrete features, such as a raised portions on a surface, depressions,or simply marks on a surface, may be used as features in themeasurements described above.

While this specification is generally centered on the metrology ofoptical components, a related class of application is the metrology ofthe molds that are used to manufacture injection-molded lenses. In thiscase, a mold exhibits all the features also found on a lens, namely anactive optical surface and one or more location, centration or alignmentdatums. The metrology steps described for one side of a lens are thenreadily applicable. For instance, the instrument is used to measure thecentration and height of the apex of the optical surface with respect tothe mechanical datums. Other metrology steps include thecharacterization of steps between outer datums, as well as the angle ofsteep conical centration datums.

In certain embodiments, such as where the part under test is larger thanthe field of view of the optical instrument, measurements of differentregions of the part may be stitched together to provide measurements ofthe entire part. Exemplary techniques for stitching measurements aredisclosed in J. Roth and P. de Groot, “Wide-field scanning white lightinterferometry of rough surfaces,” Proc. ASPE Spring Topical Meeting onAdvances in Surface Metrology, 57-60 (1997).

In some implementations, additional corrections may be applied toimprove measurement accuracy. For example, corrections for the phasechange on reflection properties of the surfaces may be applied. See,e.g., in P. de Groot, J. Biegen, J. Clark, X. Colonna de Lega and D.Grigg, “Optical Interferometry for Measurement of the GeometricDimensions of Industrial Parts,” Applied Optics 41 (19), 3853-3860(2002).

In certain implementations, the part may be measured from more than oneviewing angle, or from both sides. See, e.g., P. de Groot, J. Biegen, J.Clark, X. Colonna de Lega and D. Grigg, “Optical Interferometry forMeasurement of the Geometric Dimensions of Industrial Parts,” AppliedOptics 41 (19), 3853-3860 (2002).

The results of the measurements may be combined with other measurements,including for example stylus measurements of aspheric form, such asdisclosed, e.g., in P. Scott, “Recent Developments in the Measurement ofAspheric Surfaces by Contact Stylus Instrumentation,” 4927, 199-207(2002).

A variety of data processing methods may be applied. For instance,methods adapted to measuring multiple surfaces using coherence scanninginterferometer may be used. See, e.g., P. J. de Groot and X. Colonna deLega, “Transparent film profiling and analysis by interferencemicroscopy,” Proc. SPIE 7064, 706401-1 706401-6 (2008).

The computations associated with the measurements and analysis describedabove can be implemented in computer programs using standard programmingtechniques following the method and figures described herein. Programcode is applied to input data to perform the functions described hereinand generate output information. The output information may be appliedto one or more output devices such as a display monitor. Each programmay be implemented in a high level procedural or object orientedprogramming language to communicate with a computer system. However, theprograms can be implemented in assembly or machine language, if desired.In any case, the language can be a compiled or interpreted language.Moreover, the program can run on dedicated integrated circuitspreprogrammed for that purpose.

Each such computer program is preferably stored on a storage medium ordevice (e.g., ROM, optical disc or magnetic disc) readable by a generalor special purpose programmable computer, for configuring and operatingthe computer when the storage media or device is read by the computer toperform the procedures described herein. The computer program can alsoreside in cache or main memory during program execution. The calibrationmethod can also be implemented, at least in part, as a computer-readablestorage medium, configured with a computer program, where the storagemedium so configured causes a computer to operate in a specific andpredefined manner to perform the functions described herein.

Other embodiments are in the following claims.

1. A method for determining information about a transparent opticalelement comprising a lens portion and a plane parallel portion, the lensportion comprising at least one curved surface and the plane parallelportion comprising opposing first and second surfaces, the methodcomprising: directing measurement light to the transparent opticalelement; detecting measurement light reflected from at least onelocation on the first surface of the plane parallel portion; detectingmeasurement light reflected from the second surface of the planeparallel portion at a location corresponding to the at least onelocation on the first surface; determining, based on the detected light,information about the plane parallel portion; and evaluating thetransparent optical element based on the information about the planeparallel portion. 2-3. (canceled)
 4. The method of claim 1, wherein theinformation about the plane parallel portion comprises a height profileof the first surface of the plane parallel portion and a height profileof the second surface of the plane parallel portion.
 5. The method ofclaim 1, wherein the information about the plane parallel portioncomprises a physical thickness profile or optical thickness profile ofthe plane parallel portion.
 6. The method of claim 1, wherein theinformation about the plane parallel portion comprises information abouta refractive index of a material forming the transparent opticalelement. 7-8. (canceled)
 9. The method of claim 6, wherein theinformation about the refractive index comprises information aboutvariations of a refractive index between different locations of theplane parallel portion.
 10. The method of claim 6, wherein theinformation about the refractive index comprises information aboutbirefringence of the material forming the transparent optical element.11. The method of claim 1, further comprising detecting measurementlight reflected from a reference feature on a fixture that supports thetransparent optical element and determining, based on the detected lightfrom the reference feature information about the reference feature. 12.The method of claim 11, wherein the measurement light reflected from thereference feature is reflected from a location corresponding to the atleast one location on the first surface of the plane parallel portion.13-15. (canceled)
 16. The method of claim 1, wherein the measurementlight is detected for a first polarization and, thereafter, a secondpolarization different from the first polarization.
 17. The method ofclaim 1, wherein evaluating the transparent optical element comprisesinferring information about a dimensional or optical property of thelens portion based on the information about the plane parallel portion.18. The method of claim 1, wherein the plane parallel portion is a tiltcontrol interlock of the transparent optical element.
 19. (canceled) 20.The method of claim 1, wherein the lens portion comprises a secondcurved surface opposite the first curved surface.
 21. The method ofclaim 1, wherein the information about the lens portion comprisesinformation about birefringence of a material forming the lens portion.22. (canceled)
 23. The method of claim 1, wherein evaluating thetransparent optical element comprises determining whether thetransparent optical element meets a specification requirement based onthe information about the plane parallel portion.
 24. (canceled)
 25. Amethod of forming an optical assembly, comprising: determininginformation about the transparent optical element using the method ofclaim 1, where the transparent optical element is a lens; and securingthe lens relative to one or more other lenses in a barrel to form theoptical assembly.
 26. (canceled)
 27. A system for determininginformation about a transparent optical element comprising a lensportion and a plane parallel portion, the lens portion comprising atleast one curved surface and the plane parallel portion comprisingopposing first and second surfaces, the system comprising: a fixture forsupporting the transparent optical element; an optical instrumentcomprising a light source, a detector, and optical elements arranged todirect light from the light source towards the transparent opticalelement when the transparent optical element is supported by the fixtureand direct light reflected from the transparent optical element to thedetector; and an electronic controller in communication with thedetector, the electronic controller being programmed to determineinformation about the plane parallel portion based on light detectedfrom corresponding locations of the first and second surfaces of theplane parallel portion.
 28. The system of claim 27, wherein the opticalinstrument is an optical areal surface topography instrument. 29-30.(canceled)
 31. The system of claim 27, wherein the fixture comprises areference feature located in a path of the light from the opticalinstrument.
 32. The system of claim 31, wherein the reference feature isa planar reflector.
 33. The system of claim 31, wherein the fixturecomprises a stand which positions the transparent optical element adistance from the reference feature.
 34. The system of claim 27, whereinthe optical instrument comprises a polarization module configured topolarize light from the light source.
 35. (canceled)